The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, the approaches described in this section may not be prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
In many wireless communications applications, the available RF band is broken up into channels. For example, the 802.11g band is segmented into 14 channels (depending on the regulatory domain) which, theoretically, allows for at least three, non-overlapping channels of 20 MHz bandwidth, and thus three simultaneous non-interfering wireless links.
To exploit channelization of the band, most RF transceivers are designed for best performance (e.g. noise, linearity, interference rejection) in a single or perhaps a pair of channels. One frequently-used architecture for low-cost RF transceivers is shown in FIG. 1 (receiver only).
This so-called direct conversion receiver has an RF bandwidth that covers multiple channels. In the case of 802.11g, RF bandwidth covers approximately 2.4 GHz to 2.5 GHz. An antenna 102 receives the signal. A front-end bandpass filter (BPF) 104 limits the RF band signal. The band-limited RF signal is amplified by a low-noise amplifier (LNA) 106 prior to a frequency shift to a baseband (BB) frequency using a mixer 110 receiving input from synthesizer 108. The final bandwidth of the receiver is defined by a pair of lowpass filters (BB LPF) 112. The baseband signal is further amplified by a variable-gain amplifier (VGA) 114. Control signals 116 can be applied to one or more of the LNA 106, synthesizer 108, BB LPF 112 and VGA 114 to adjust performance.
For the architecture illustrated in FIG. 1, the receiver's front-end RF bandwidth is set to cover the entire available band, while the BB bandwidth is set to cover (typically) one channel only. In an RF monitoring or scanning scenario, the entire RF bandwidth needs to be analyzed. Consequently, the receiver must scan the channels in some way, dwelling on a channel for some specified amount of time. Typically, the scanning would take place as shown in FIG. 2.
For simplicity, the channels are shown to be disjoint with no gaps between them. Also for simplicity, the time to complete a channel change is assumed to be zero. The dwell time in each channel is the same.
In the channel configuration illustrated in the drawings, five channels are required to cover the RF band. Consequently, when the dwell time in each channel is the same, the duty factor for analysis in each channel is 1/5. Continuous scanning would be accomplished by repeating the channel sequence.
If there is no a priori indication of a signal-of-interest (SOI), specifically no knowledge of what channel in the band that the SOI would be likely to occur, then the scanning shown in FIG. 2 would be adequate.
If a SOI is suspected to occupy one or more channels more frequently than others, then the scanning shown in FIG. 2 could be modified to dwell on the more likely channel(s) for a longer period of time. Dwelling on a more likely channel for a longer period of time provides more opportunities to observe the signal (for example, a pulsed signal) and/or better estimation of signal features due to a longer observation time.
Examples are shown in FIGS. 3 and 4. FIG. 3 shows a scanning protocol that dwells longer on channel C2 and shorter on C3. FIG. 4 shows non-sequential channel scanning in which the scanning sequence is {C0, C1, C0, C2, C4, C3}.